The term “industrial filtration fabric” as used herein refers to woven fabrics such as are used to drain, form or otherwise consolidate a dispersion or dilute slurry of fibers or similar solids into a somewhat cohesive mat or web. Such fabrics provide a moving support surface to receive the initial deposit of the dispersion or slurry, and carry or otherwise support it for a suitable distance. Apertures through the fabric can provide drainage for liquids, while the incipient cohesive mat or web of solids is being formed.
One type of industrial filtration fabric, for which the present invention is particularly applicable, is a “papermakers fabric”. This term refers to industrial textiles that are used in the process of making paper and similar sheet products; and such fabrics include forming fabrics, press felts and dryer fabrics. The present invention is particularly relevant to papermakers' forming fabrics which are used to form a continuous web of cellulosic fibers.
In relation to these industrial filtration fabrics, the term “paper side” or PS refers to the substantially planar surface of the fabric upon which the cohesive mat or web is formed or, in the later stages, transported. The term “machine side” or MS refers to the surface which is located opposite the paper side and is generally in moving contact with various stationary elements of the machine in which it is used, such as the drainage elements, rolls, foils and blades of a papermaking machine. In the discussion below, the novel fabrics of the present invention are described primarily in the context of the papermaking process, but it is to be understood that the invention is not so limited and the invention will find utility in numerous other specific industrial filtration applications, including pulp dewatering and pulp cake formation, sewage treatment, nonwovens formation and conveyance, and the like.
In the papermaking process, a very dilute slurry of about 1% papermaking fibers together with a mixture of about 99% water and other papermaking components is ejected at high speed and precision from the slice opening of a headbox on to the PS of a moving forming fabric. The fabric is guided and driven by a number of rolls over various drainage boxes and foils which assist in the removal of water through the fabric so as to leave behind a randomly dispersed, loosely cohesive network or web of papermaking fibers. At the end of the forming section, this web is transferred to the press section, where further water removal occurs by mechanical pressures as the web is conveyed on or between a series of press fabrics and is guided through one or more nips. The now self-supporting but still very wet web is then transferred to the dryer section of the papermaking machine where the remaining water is removed by evaporation. The resulting paper product may then be exposed to various treatments before it is then finally wound onto a reel, cut to size and packaged for shipment.
It is widely acknowledged that the forming fabric plays a critical role in the initial formation of the paper web. The forming fabric is required to simultaneously satisfy a number of physical requirements. It must be rugged, so as to withstand over time the continuous moving contact to which its lower (machine side) surface is exposed as it is driven over the various stationary contact surfaces in the forming section. It must be stable, so that it does not crease or skew during operation. At the same time, it must provide an appropriate PS surface, which for smooth paper products is required to be very fine, upon which the individual fibers in the stock slurry are deposited, along with any added fines and fillers, so as to form a planar web which will eventually be consolidated into a continuous sheet following water removal in the downstream sections of the papermaking machine. The fineness of the fabric used in the papermaking process (i.e. the size of the yarns, openings in the mesh and number of support points per unit area provided by the fabric) will be dictated partly by the length of the papermaking fibers used in the stock and partly by the end use requirements of the paper product being formed.
It is also known that increasing the fineness of the yarns for the fabric, i.e. by reducing their diameter, together with increasing the yarn count, provides increased support for shorter papermaking fibers; however, this leads to problems in providing sufficient mechanical stability for the fabric. A further result of using smaller diameter yarns is that it can provide a more open fabric structure, so that the sheet will be dryer on leaving the couch at the end of the forming section.
Papermaking fibers are increasingly derived from recycled materials, and such fibers are generally shorter in length than fibers obtained from virgin sources, e.g. 0.5-1.5 mm for recycle fibers, in contrast with 2-4 mm for virgin. Papermaking stocks increasingly contain significant percentages of such recycled fibers which must be supported by the mesh of the fabric upon which they are deposited if they are to provide benefit in the papermaking process. Increased support for the papermaking fibers can only be provided by decreasing the cross-section area of the yarns from which the fabric is woven, and increasing the mesh (i.e. the density or number of yarns in each fabric direction). A fine mesh will provide more support points for the papermaking fibers, but a fine mesh will also result in a woven structure that is less rugged than a comparable fabric that is woven using larger yarns. Thus, the use of finer yarns in these fabrics has resulted in thinner fabric structures which are less mechanically stable and have reduced wear capability, leading to the need to find other means of providing the required stability and wear capability.
A further problem common to all papermaking machines and which can have an adverse effect on the formation properties of the web is the problem of “impingement drainage”, i.e. the drainage of excessive amounts of the stock into or through the moving forming fabric at or close to the point of impingement on the fabric, as discussed below.
In the initial portion of the forming section (either with or without an initial open surface portion), an unsupported jet of highly aqueous stock is ejected at high speed from the head box slice onto the open PS surface of a moving forming fabric, or into the more or less convergent wedge shaped space between two moving forming fabrics in the case of a twin wire former. The jet will typically traverse a short distance before impinging the PS surface of the forming fabric, or fabrics, at the point of impingement. The angle of impingement formed between the linear axis of the stock jet and the PS surface of the forming fabric, or fabrics, on which paper is made is generally quite small. In Fourdrinier machines, the angle was typically up to about 4° ′ but up to about 6° for blade gap formers, and in modern roll gap formers, the angle may be up to about 10°. Since the angle of impingement cannot be zero, i.e. tangential to the fabric surface, or fabric surfaces in a twin fabric paper making machine, at least in part because the stock jet widens in the direction perpendicular to the fabric surface or surfaces in the space between the head box slice and the point of impingement, the pressure exerted by the stock jet onto the forming fabric or fabrics can be resolved into two components: a component essentially tangential to the fabric surface, and a component essentially perpendicular to the fabric surface, both of which when combined have a considerable effect on impingement drainage rates.
This problem of impingement drainage, and the particularly increased significance in relation to machines operated at higher speeds and with a greater angle of impingement, is discussed in Danby, Roger and Dale Johnson, Float Forming, Proceedings, PAPTAC 92nd Annual Meeting 2006, Montreal, QC, February 2006, pp. C141-C148.
As noted by those authors, the higher the angle of impingement, the greater will be the impingement velocity. These forces are directly proportional to the speed at which the forming fabric moves in the machine direction: i.e. as the machine speed increases so do the impingement forces. A further factor which affects the impingement characteristics is the mechanical geometry of the papermaking machine itself. As noted above, modern roll formers which are used on newer papermaking machines have greater impingement angles than in most previously used forming section constructions, and thus greater impingement velocities, which in turn lead to problems including poorer fines retention, sheet embedment, and generally reduced quality of formation.
In modern high speed papermaking machines in which the forming fabric(s) can be moving at speeds of 100 kph, or more, the minor pressure component perpendicular to the fabric surface exerts a significant level of force on the forming fabric, which can cause excessive impingement derived drainage of the stock over the initial portion of the forming section. This pressure component (the “impingement pressure”), which is minor on some machines but of increasing significance in many newer machines, and the turbulent forces created by stationary drainage elements, combined with the increased use of particulate fillers and shorter papermaking fibres, have the undesirable effect of reducing first pass retention and increasing the embedment of the initial layers of the embryonic web into the paper side surface of the forming fabric.
It is well known that, on any papermaking machine under start up conditions and delivering a normal papermaking volume of water but without papermaking fibers from the headbox slice onto the forming fabric, this water will drain within a very short distance, approximately 12 inches (30 cm), or less than 1% of the total available drainage length of the typical forming section. This indicates that, without fibers, all forming fabrics have far in excess of the drainage capacity required to make paper. However, as soon as papermaking fibers are introduced, drainage is retarded at a rate determined by the length of the fibers, the quantity of fibers, the support characteristics of the papermaking surface of the forming fabric, and by the forces resisting and retarding impingement drainage. It was for this reason that the original forming boards installed on open surface Fourdrinier type papermaking machines were so successful. In some more modern twin wire formers such as gap formers, the impingement shoe serves that function. However, impingement shoes cannot be used in roll formers, due to the lack of space resulting from their mechanical configuration. This is a significant problem, in that the majority of new machines are roll formers.
It is also well known that impingement drainage can cause sheet marking, low retention by the forming fabric of papermaking fibres, fines and fillers (i.e. low first pass retention), and plugging (i.e. sheet sealing) of the paper side layer of the forming fabric. Unless the structure of the forming fabric is designed to allow it to manage and control impingement drainage, further increases in machine speed and/or paper making machine efficiency may be limited or, in the case of gap formers, tied directly to improvements in forming shoe or forming board construction.
Similarly, for other industrial filtration purposes as noted above, impingement drainage will have adverse effects on the efficiency of the filtration fabric in achieving the particular purpose for which it is being used. The shorter the fibers, and the lower the fiber support from the fabric, the lower the filtration efficiency.
The future demands of the paper industry will undoubtedly be towards ever lighter basis weight sheets which will be required to be made with ever decreasing fiber lengths due to recycling, at much greater paper machine speeds in order to reduce manufacturing costs. In order to achieve this, finer papermaking fabric structures will be required than are currently available, which will be woven or otherwise assembled using yarns of increasingly smaller cross-sectional area. The resulting fabric structures will be thinner and less stable than those woven using relatively larger size yarns. If such increases in paper machine speeds and the mechanical design of the newer high speed paper machines are to be accommodated, this will require much greater fabric stability, especially in the cross machine direction, in order to produce a uniform basis weight sheet of paper.
There is therefore a need for fabric weave designs to meet these new requirements, and the problems of decreased fiber lengths, and to overcome the disadvantages, discussed above, of the use of finer yarns.